PIK3C2B inhibition improves function and prolongs survival in myotubular myopathy animal models

A healthcare claims analysis to identify and characterize patients with suspected X-Linked Myotubular Myopathy (XLMTM) in the Brazilian Healthcare System

BACKGROUND

X-linked myotubular myopathy (XLMTM) is a rare, life-threatening congenital disease, which is not well-defined. To our knowledge, no studies characterizing the XLMTM disease burden have been conducted in Brazil. We identified and described patients with suspected XLMTM using administrative claims data from the Brazilian public healthcare system.

METHODS

Data from 2015 to 2019 were extracted from the DATASUS database. As no XLMTM-specific ICD-10 code was available, a stepwise algorithm was applied to identify patients with suspected XLMTM by selecting male patients with a congenital myopathies code (G71.2), aged < 18 years at index date (first claim of G71.2), with an associated diagnostic procedure (muscle biopsy/genetic test) and without spinal muscular atrophy or Duchenne muscular dystrophy. We attempted to identify patients with suspected severe XLMTM based on use of both respiratory and feeding support, which are nearly universal in the care of XLMTM patients. Analyses were performed for the overall cohort and stratified by age at index date < 5 years old and ≥ 5 years old.

RESULTS

Of 173 patients with suspected XLMTM identified, 39% were < 5 years old at index date. Nearly all (N = 166) patients (96%) were diagnosed by muscle biopsy (91% of patients < 5 years old and 99% of patients ≥ 5 years old), six (3.5%) were diagnosed by clinical evaluation (8% of patients < 5 years old and 1% of patients ≥ 5 years old), and one was diagnosed by a genetic test. Most patients lived in Brasilia (n = 55), São Paulo (n = 33) and Minas Gerais (n = 27). More than 85% of patients < 5 years old and approximately 75% of patients ≥ 5 years old had physiotherapy at the index date. In both age groups, nearly 50% of patients required hospitalization at some point and 25% required mobility support. Respiratory and feeding support were required for 3% and 12% of patients, respectively, suggesting that between 5 and 21 patients may have had severe XLMTM.

CONCLUSION

In this real-world study, genetic testing for XLMTM appears to be underutilized in Brazil and may contribute to underdiagnosis of the disease. Access to diagnosis and care is limited outside of specific regions with specialized clinics and hospitals. Substantial use of healthcare resources included hospitalization, physiotherapy, mobility support, and, to a lesser extent, feeding support and respiratory support.

Phosphoinositide switches in cell physiology - From molecular mechanisms to disease

Phosphoinositides are amphipathic lipid molecules derived from phosphatidylinositol that represent low abundance components of biological membranes. Rather than serving as mere structural elements of lipid bilayers, they represent molecular switches for a broad range of biological processes, including cell signaling, membrane dynamics and remodeling, and many other functions. Here, we focus on the molecular mechanisms that turn phosphoinositides into molecular switches and how the dysregulation of these processes can lead to disease.

Standardization of zebrafish drug testing parameters for muscle diseases

Skeletal muscular diseases predominantly affect skeletal and cardiac muscle, resulting in muscle weakness, impaired respiratory function and decreased lifespan. These harmful outcomes lead to poor health-related quality of life and carry a high healthcare economic burden. The absence of promising treatments and new therapies for muscular disorders requires new methods for candidate drug identification and advancement in animal models. Consequently, the rapid screening of drug compounds in an animal model that mimics features of human muscle disease is warranted. Zebrafish are a versatile model in preclinical studies that support developmental biology and drug discovery programs for novel chemical entities and repurposing of established drugs. Due to several advantages, there is an increasing number of applications of the zebrafish model for high-throughput drug screening for human disorders and developmental studies. Consequently, standardization of key drug screening parameters, such as animal husbandry protocols, drug compound administration and outcome measures, is paramount for the continued advancement of the model and field. Here, we seek to summarize and explore critical drug treatment and drug screening parameters in the zebrafish-based modeling of human muscle diseases. Through improved standardization and harmonization of drug screening parameters and protocols, we aim to promote more effective drug discovery programs.

[PI3KC2β: A promising therapeutic target in myotubular myopathy]

Myotubular myopathy is a rare disease of genetic origin characterized by significant muscle weakness leading to respiratory disorders and for which no treatment exists today. In this paper, we show that inhibition of the activity of the enzyme PI3KC2β prevents the development of this myopathy in a mouse model of the disease, thus identifying a therapeutic target to treat myotubular myopathy in humans.

Loss of Mtm1 causes cholestatic liver disease in a model of X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM) is a fatal congenital disorder caused by mutations in the MTM1 gene. Currently, there are no approved treatments, although AAV8-mediated gene transfer therapy has shown promise in animal models and preliminarily in patients. However, 4 patients with XLMTM treated with gene therapy have died from progressive liver failure, and hepatobiliary disease has now been recognized more broadly in association with XLMTM. In an attempt to understand whether loss of MTM1 itself is associated with liver pathology, we have characterized what we believe to be a novel liver phenotype in a zebrafish model of this disease. Specifically, we found that loss-of-function mutations in mtm1 led to severe liver abnormalities including impaired bile flux, structural abnormalities of the bile canaliculus, and improper endosome-mediated trafficking of canalicular transporters. Using a reporter-tagged Mtm1 zebrafish line, we established localization of Mtm1 in the liver in association with Rab11, a marker of recycling endosomes, and canalicular transport proteins and demonstrated that hepatocyte-specific reexpression of Mtm1 could rescue the cholestatic phenotype. Last, we completed a targeted chemical screen and found that Dynasore, a dynamin-2 inhibitor, was able to partially restore bile flow and transporter localization to the canalicular membrane. In summary, we demonstrate, for the first time to our knowledge, liver abnormalities that were directly caused by MTM1 mutation in a preclinical model, thus establishing the critical framework for better understanding and comprehensive treatment of the human disease.

Real-world analysis of healthcare resource utilization by patients with X-linked myotubular myopathy (XLMTM) in the United States

BACKGROUND

X-linked myotubular myopathy (XLMTM) is a rare, life-threatening congenital myopathy with multisystem involvement, often requiring invasive ventilator support, gastrostomy tube feeding, and wheelchair use. Understanding healthcare resource utilization in patients with XLMTM is important for development of targeted therapies but data are limited.

METHODS

We analyzed individual medical codes as governed by Healthcare Common Procedure Coding System, Current Procedural Terminology, and International Classification of Diseases, 10th Revision (ICD-10) for a defined cohort of XLMTM patients within a US medical claims database. Using third-party tokenization software, we defined a cohort of XLMTM patient tokens from a de-identified dataset in a research registry of diagnostically confirmed XLMTM patients and de-identified data from a genetic testing company. After approval of an ICD-10 diagnosis code for XLMTM (G71.220) in October 2020, we identified additional patients.

RESULTS

A total of 192 males with a diagnosis of XLMTM were included: 80 patient tokens and 112 patients with the new ICD-10 code. From 2016 to 2020, the annual number of patients with claims increased from 120 to 154 and the average number of claims per patient per year increased from 93 to 134. Of 146 patients coded with hospitalization claims, 80 patients (55%) were first hospitalized between 0 and 4 years of age. Across all patients, 31% were hospitalized 1-2 times, 32% 3-9 times, and 14% ≥ 10 times. Patients received care from multiple specialty practices: pulmonology (53%), pediatrics (47%), neurology (34%), and critical care medicine (31%). The most common conditions and procedures related to XLMTM were respiratory events (82%), ventilation management (82%), feeding difficulties (81%), feeding support (72%), gastrostomy (69%), and tracheostomy (64%). Nearly all patients with respiratory events had chronic respiratory claims (96%). The most frequent diagnostic codes were those investigating hepatobiliary abnormalities.

CONCLUSIONS

This innovative medical claims analysis shows substantial healthcare resource use in XLMTM patients that increased over the last 5 years. Most patients required respiratory and feeding support and experienced multiple hospitalizations throughout childhood and beyond for those that survived. This pattern delineation will inform outcome assessments with the emergence of novel therapies and supportive care measures.

Integration of Selection Signatures and Protein Interactions Reveals NR6A1, PAPPA2, and PIK3C2B as the Promising Candidate Genes Underlying the Characteristics of Licha Black Pig

Licha black (LI) pig has the specific characteristics of larger body length and appropriate fat deposition among Chinese indigenous pigs. Body length is one of the external traits that affect production performance, and fat deposition influences meat quality. However, the genetic characteristics of LI pigs have not yet been systematically uncovered. Here, the genomic information from 891 individuals of LI pigs, commercial pigs, and other Chinese indigenous pigs was used to analyze the breed characteristics of the LI pig with runs of homozygosity, haplotype, and FST selection signatures. The results showed the growth traits-related genes (i.e., NR6A1 and PAPPA2) and the fatness traits-related gene (i.e., PIK3C2B) were the promising candidate genes that closely related to the characteristics of LI pigs. In addition, the protein–protein interaction network revealed the potential interactions between the promising candidate genes and the FASN gene. The RNA expression data from FarmGTEx indicated that the RNA expression levels of NR6A1, PAPPA2, PIK3C2B, and FASN were highly correlated in the ileum. This study provides valuable molecular insights into the mechanisms that affect pig body length and fat deposition, which can be used in the further breeding process to improve meat quality and commercial profitability.

Therapeutic approaches in different congenital myopathies

Congenital myopathies are rare and severe genetic diseases affecting the skeletal muscle function in children and adults. They present a variable spectrum of phenotypes and a genetic heterogeneity. Subgroups are defined according to the clinical and histopathological features and encompass core myopathy, centronuclear myopathy, nemaline myopathy and other rare congenital myopathies. No approved treatment exists to date for any congenital myopathies. To tackle this important unmet need, an increased number of proof-of-concept studies recently assessed the therapeutic potential of various strategies, either pharmacological or genetic-based, aiming at counteracting muscle weakness or/and cure the pathology. Here, we list the implicated genes and cellular pathways, and review the therapeutic approaches preclinically tested and the ongoing/completed clinical trials for the different types of congenital myopathies.

Congenital myopathies

The congenital myopathies are inherited muscle disorders characterized clinically by hypotonia and weakness, usually from birth, with a static or slowly progressive clinical course. Historically, the congenital myopathies have been classified according to major morphological features seen on muscle biopsy as nemaline myopathy, central core disease, centronuclear or myotubular myopathy, and congenital fiber type disproportion. However, in the past two decades, the genetic basis of these different forms of congenital myopathy has been further elucidated with the result being improved correlation with histological and genetic characteristics. However, these notions have been challenged for three reasons. First, many of the congenital myopathies can be caused by mutations in more than one gene that suggests an impact of genetic heterogeneity. Second, mutations in the same gene can cause different muscle pathologies. Third, the same genetic mutation may lead to different pathological features in members of the same family or in the same individual at different ages. This chapter provides a clinical overview of the congenital myopathies and a clinically useful guide to its genetic basis recognizing the increasing reliance of exome, subexome, and genome sequencing studies as first-line analysis in many patients.

Beyond PI3Ks: targeting phosphoinositide kinases in disease

Lipid phosphoinositides are master regulators of almost all aspects of a cell's life and death and are generated by the tightly regulated activity of phosphoinositide kinases. Although extensive efforts have focused on drugging class I phosphoinositide 3-kinases (PI3Ks), recent years have revealed opportunities for targeting almost all phosphoinositide kinases in human diseases, including cancer, immunodeficiencies, viral infection and neurodegenerative disease. This has led to widespread efforts in the clinical development of potent and selective inhibitors of phosphoinositide kinases. This Review summarizes our current understanding of the molecular basis for the involvement of phosphoinositide kinases in disease and assesses the preclinical and clinical development of phosphoinositide kinase inhibitors.

Antagonistic control of active surface integrins by myotubularin and phosphatidylinositol 3-kinase C2β in a myotubular myopathy model

X-linked centronuclear myopathy (XLCNM) is a severe human disease without existing therapies caused by mutations in the phosphoinositide 3-phosphatase MTM1. Loss of MTM1 function is associated with muscle fiber defects characterized by impaired localization of β-integrins and other components of focal adhesions. Here we show that defective focal adhesions and reduced active β-integrin surface levels in a cellular model of XLCNM are rescued by loss of phosphatidylinositiol 3-kinase C2β (PI3KC2β) function. Inactivation of the Mtm1 gene impaired myoblast differentiation into myotubes and resulted in reduced surface levels of active β1-integrins as well as corresponding defects in focal adhesions. These phenotypes were rescued by concomitant genetic loss of Pik3c2b or pharmacological inhibition of PI3KC2β activity. We further demonstrate that a hitherto unknown role of PI3KC2β in the endocytic trafficking of active β1-integrins rather than rescue of phosphatidylinositol 3-phosphate levels underlies the ability of Pik3c2b to act as a genetic modifier of cellular XLCNM phenotypes. Our findings reveal a crucial antagonistic function of MTM1 and PI3KC2β in the control of active β-integrin surface levels, thereby providing a molecular mechanism for the adhesion and myofiber defects observed in XLCNM. They further suggest specific pharmacological inhibition of PI3KC2β catalysis as a viable treatment option for XLCNM patients.

X-linked myotubular myopathy is associated with epigenetic alterations and is ameliorated by HDAC inhibition

X-linked myotubular myopathy (XLMTM) is a fatal neuromuscular disorder caused by loss of function mutations in MTM1. At present, there are no directed therapies for XLMTM, and incomplete understanding of disease pathomechanisms. To address these knowledge gaps, we performed a drug screen in mtm1 mutant zebrafish and identified four positive hits, including valproic acid, which functions as a potent suppressor of the mtm1 zebrafish phenotype via HDAC inhibition. We translated these findings to a mouse XLMTM model, and showed that valproic acid ameliorates the murine phenotype. These observations led us to interrogate the epigenome in Mtm1 knockout mice; we found increased DNA methylation, which is normalized with valproic acid, and likely mediated through aberrant 1-carbon metabolism. Finally, we made the unexpected observation that XLMTM patients share a distinct DNA methylation signature, suggesting that epigenetic alteration is a conserved disease feature amenable to therapeutic intervention.

Clinical Characteristics and Neurologic Outcomes of X-Linked Myotubular Myopathy

Purpose: X-linked myotubular myopathy (XLMTM) is a rare condition of centronuclear myopathy caused by myotubularin 1 (MTM1) mutations. Patients with XLMTM show different neurodevelopmental outcomes after the neonatal period depending on age and acquired hypoxic damage. We aim to evaluate the clinical characteristics and neurodevelopmental outcomes of patients with XLMTM who were followed up at a single center. It is essential to understand the volume and conditions to prepare for being a candidate for new therapeutic strategies. Methods: Patients diagnosed with centronuclear myopathy by muscle pathology and MTM1 mutation analysis were included. We retrospectively investigated motor milestones, communication skills, and bulbar and respiratory function in the patients. The patients were categorized into two groups: with and without hypoxic insults (HI). Results: All 13 patients were severely affected by neonatal hypotonia and required respiratory support and a feeding tube during the neonatal period. The follow-up duration was 4.4 years (range, 0.3 to 8.9). In the non-HI group, developmental milestones were delayed but were slowly achieved. Some patients underwent training in oral feeding with thickened foods and weaning from ventilation. Patients with HI showed poor motor function catch-up and communication skills. Three deaths were associated with acute respiratory failure.Conclusion: Patients with XLMTM without HI can survive long-term with the slow achievement of motor milestones and bulbar and respiratory function. However, hypoxic brain damage following acute respiratory failure negatively influences their developmental potential or even lead to death. Therefore, parental education for proper respiratory management is necessary, especially for young children.

Dynamin-2 reduction rescues the skeletal myopathy of SPEG-deficient mouse model

Striated preferentially expressed protein kinase (SPEG), a myosin light chain kinase, is mutated in centronuclear myopathy (CNM) and/or dilated cardiomyopathy. No precise therapies are available for this disorder, and gene replacement therapy is not a feasible option due to the large size of SPEG. We evaluated the potential of dynamin-2 (DNM2) reduction as a potential therapeutic strategy because it has been shown to revert muscle phenotypes in mouse models of CNM caused by MTM1, DNM2, and BIN1 mutations. We determined that SPEG-β interacted with DNM2, and SPEG deficiency caused an increase in DNM2 levels. The DNM2 reduction strategy in Speg-KO mice was associated with an increase in life span, body weight, and motor performance. Additionally, it normalized the distribution of triadic proteins, triad ultrastructure, and triad number and restored phosphatidylinositol-3-phosphate levels in SPEG-deficient skeletal muscles. Although DNM2 reduction rescued the myopathy phenotype, it did not improve cardiac dysfunction, indicating a differential tissue-specific function. Combining DNM2 reduction with other strategies may be needed to target both the cardiac and skeletal defects associated with SPEG deficiency. DNM2 reduction should be explored as a therapeutic strategy against other genetic myopathies (and dystrophies) associated with a high level of DNM2.

Natural history of a mouse model of X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM) is a severe monogenetic disorder of the skeletal muscle. It is caused by loss-of-expression/function mutations in the myotubularin (MTM1) gene. Much of what is known about the disease, as well as the treatment strategies, has been uncovered through experimentation in pre-clinical models, particularly the Mtm1 gene knockout mouse line (Mtm1 KO). Despite this understanding, and the identification of potential therapies, much remains to be understood about XLMTM disease pathomechanisms, and about the normal functions of MTM1 in muscle development. To lay the groundwork for addressing these knowledge gaps, we performed a natural history study of Mtm1 KO mice. This included longitudinal comparative analyses of motor phenotype, transcriptome and proteome profiles, muscle structure and targeted molecular pathways. We identified age-associated changes in gene expression, mitochondrial function, myofiber size and key molecular markers, including DNM2. Importantly, some molecular and histopathologic changes preceded overt phenotypic changes, while others, such as triad structural alternations, occurred coincidentally with the presence of severe weakness. In total, this study provides a comprehensive longitudinal evaluation of the murine XLMTM disease process, and thus provides a critical framework for future investigations.

Summary: This study provides a comprehensive and longitudinal molecular and phenotypic evaluation of the disease process of X-linked myotubular myopathy (XLMTM) in a murine model.

Variants in ASPH cause exertional heat illness and are associated with malignant hyperthermia susceptibility

Exertional heat illness (EHI) and malignant hyperthermia (MH) are life threatening conditions associated with muscle breakdown in the setting of triggering factors including volatile anesthetics, exercise, and high environmental temperature. To identify new genetic variants that predispose to EHI and/or MH, we performed genomic sequencing on a cohort with EHI/MH and/or abnormal caffeine-halothane contracture test. In five individuals, we identified rare, pathogenic heterozygous variants in ASPH, a gene encoding junctin, a regulator of excitation-contraction coupling. We validated the pathogenicity of these variants using orthogonal pre-clinical models, CRISPR-edited C2C12 myotubes and transgenic zebrafish. In total, we demonstrate that ASPH variants represent a new cause of EHI and MH susceptibility.

Characterization of a novel zebrafish model of SPEG-related centronuclear myopathy

Centronuclear myopathy (CNM) is a congenital neuromuscular disorder caused by pathogenic variation in genes associated with membrane trafficking and excitation–contraction coupling (ECC). Bi-allelic autosomal-recessive mutations in striated muscle enriched protein kinase (SPEG) account for a subset of CNM patients. Previous research has been limited by the perinatal lethality of constitutive Speg knockout mice. Thus, the precise biological role of SPEG in developing skeletal muscle remains unknown. To address this issue, we generated zebrafish spega, spegb and spega;spegb (speg-DKO) mutant lines. We demonstrated that speg-DKO zebrafish faithfully recapitulate multiple phenotypes associated with CNM, including disruption of the ECC machinery, dysregulation of calcium homeostasis during ECC and impairment of muscle performance. Taking advantage of zebrafish models of multiple CNM genetic subtypes, we compared novel and known disease markers in speg-DKO with mtm1-KO and DNM2-S619L transgenic zebrafish. We observed Desmin accumulation common to all CNM subtypes, and Dnm2 upregulation in muscle of both speg-DKO and mtm1-KO zebrafish. In all, we establish a new model of SPEG-related CNM, and identify abnormalities in this model suitable for defining disease pathomechanisms and evaluating potential therapies.

This article has an associated First Person interview with the joint first authors of the paper.

Summary: We created a novel zebrafish Speg mutant model of centronuclear myopathy that recapitulates key features of the human disorder and provides insight into pathomechanisms of the disease.

INCEPTUS Natural History, Run-in Study for Gene Replacement Clinical Trial in X-Linked Myotubular Myopathy

BACKGROUND

X-linked myotubular myopathy (XLMTM) is a life-threatening congenital myopathy that, in most cases, is characterized by profound muscle weakness, respiratory failure, need for mechanical ventilation and gastrostomy feeding, and early death.

OBJECTIVE

We aimed to characterize the neuromuscular, respiratory, and extramuscular burden of XLMTM in a prospective, longitudinal study.

METHODS

Thirty-four participants < 4 years old with XLMTM and receiving ventilator support enrolled in INCEPTUS, a prospective, multicenter, non-interventional study. Disease-related adverse events, respiratory and motor function, feeding, secretions, and quality of life were assessed.

RESULTS

During median (range) follow-up of 13.0 (0.5, 32.9) months, there were 3 deaths (aspiration pneumonia; cardiopulmonary failure; hepatic hemorrhage with peliosis) and 61 serious disease-related events in 20 (59%) participants, mostly respiratory (52 events, 18 participants). Most participants (80%) required permanent invasive ventilation (>16 hours/day); 20% required non-invasive support (6-16 hours/day). Median age at tracheostomy was 3.5 months (95% CI: 2.5, 9.0). Thirty-three participants (97%) required gastrostomy. Thirty-one (91%) participants had histories of hepatic disease and/or prospectively experienced related adverse events or laboratory or imaging abnormalities. CHOP INTEND scores ranged from 19-52 (mean: 35.1). Seven participants (21%) could sit unsupported for≥30 seconds (one later lost this ability); none could pull to stand or walk with or without support. These parameters remained static over time across the INCEPTUS cohort.

CONCLUSIONS

INCEPTUS confirmed high medical impact, static respiratory, motor and feeding difficulties, and early death in boys with XLMTM. Hepatobiliary disease was identified as an under-recognized comorbidity. There are currently no approved disease-modifying treatments.

OUP accepted manuscript

Epilepsy is one of the most frequent neurological diseases, with focal epilepsy accounting for the largest number of cases. The genetic alterations involved in focal epilepsy are far from being fully elucidated.

Here, we show that defective lipid signalling caused by heterozygous ultra-rare variants in PIK3C2B, encoding for the class II phosphatidylinositol 3-kinase PI3K-C2β, underlie focal epilepsy in humans. We demonstrate that patients’ variants act as loss-of-function alleles, leading to impaired synthesis of the rare signalling lipid phosphatidylinositol 3,4-bisphosphate, resulting in mTORC1 hyperactivation. In vivo, mutant Pik3c2b alleles caused dose-dependent neuronal hyperexcitability and increased seizure susceptibility, indicating haploinsufficiency as a key driver of disease. Moreover, acute mTORC1 inhibition in mutant mice prevented experimentally induced seizures, providing a potential therapeutic option for a selective group of patients with focal epilepsy.

Our findings reveal an unexpected role for class II PI3K-mediated lipid signalling in regulating mTORC1-dependent neuronal excitability in mice and humans.

Phosphoinositides as membrane organizers

Phosphoinositides are signalling lipids derived from phosphatidylinositol, a ubiquitous phospholipid in the cytoplasmic leaflet of eukaryotic membranes. Initially discovered for their roles in cell signalling, phosphoinositides are now widely recognized as key integrators of membrane dynamics that broadly impact on all aspects of cell physiology and on disease. The past decade has witnessed a vast expansion of our knowledge of phosphoinositide biology. On the endocytic and exocytic routes, phosphoinositides direct the inward and outward flow of membrane as vesicular traffic is coupled to the conversion of phosphoinositides. Moreover, recent findings on the roles of phosphoinositides in autophagy and the endolysosomal system challenge our view of lysosome biology. The non-vesicular exchange of lipids, ions and metabolites at membrane contact sites in between organelles has also been found to depend on phosphoinositides. Here we review our current understanding of how phosphoinositides shape and direct membrane dynamics to impact on cell physiology, and provide an overview of emerging concepts in phosphoinositide regulation.

Activation Mechanisms of the VPS34 Complexes

Phosphatidylinositol-3-phosphate (PtdIns(3)P) is essential for cell survival, and its intracellular synthesis is spatially and temporally regulated. It has major roles in two distinctive cellular pathways, namely, the autophagy and endocytic pathways. PtdIns(3)P is synthesized from phosphatidylinositol (PtdIns) by PIK3C3C/VPS34 in mammals or Vps34 in yeast. Pathway-specific VPS34/Vps34 activity is the consequence of the enzyme being incorporated into two mutually exclusive complexes: complex I for autophagy, composed of VPS34/Vps34-Vps15/Vps15-Beclin 1/Vps30-ATG14L/Atg14 (mammals/yeast), and complex II for endocytic pathways, in which ATG14L/Atg14 is replaced with UVRAG/Vps38 (mammals/yeast). Because of its involvement in autophagy, defects in which are closely associated with human diseases such as cancer and neurodegenerative diseases, developing highly selective drugs that target specific VPS34/Vps34 complexes is an essential goal in the autophagy field. Recent studies on the activation mechanisms of VPS34/Vps34 complexes have revealed that a variety of factors, including conformational changes, lipid physicochemical parameters, upstream regulators, and downstream effectors, greatly influence the activity of these complexes. This review summarizes and highlights each of these influences as well as clarifying key questions remaining in the field and outlining future perspectives.

X-linked myotubular myopathy

X-linked myotubular myopathy (XLMTM) is a severe congenital muscle disease caused by mutation in the MTM1 gene. MTM1 encodes myotubularin (MTM1), an endosomal phosphatase that acts to dephosphorylate key second messenger lipids PI3P and PI3,5P2. XLMTM is clinically characterized by profound muscle weakness and associated with multiple disabilities (including ventilator and wheelchair dependence) and early death in most affected individuals. The disease is classically defined by characteristic changes observed on muscle biopsy, including centrally located nuclei, myofiber hypotrophy, and organelle disorganization. In this review, we highlight the clinical and pathologic features of the disease, present concepts related to disease pathomechanisms, and present recent advances in therapy development.

Common Pathogenic Mechanisms in Centronuclear and Myotubular Myopathies and Latest Treatment Advances

Centronuclear myopathies (CNM) are rare congenital disorders characterized by muscle weakness and structural defects including fiber hypotrophy and organelle mispositioning. The main CNM forms are caused by mutations in: the MTM1 gene encoding the phosphoinositide phosphatase myotubularin (myotubular myopathy), the DNM2 gene encoding the mechanoenzyme dynamin 2, the BIN1 gene encoding the membrane curvature sensing amphiphysin 2, and the RYR1 gene encoding the skeletal muscle calcium release channel/ryanodine receptor. MTM1, BIN1, and DNM2 proteins are involved in membrane remodeling and trafficking, while RyR1 directly regulates excitation-contraction coupling (ECC). Several CNM animal models have been generated or identified, which confirm shared pathological anomalies in T-tubule remodeling, ECC, organelle mispositioning, protein homeostasis, neuromuscular junction, and muscle regeneration. Dynamin 2 plays a crucial role in CNM physiopathology and has been validated as a common therapeutic target for three CNM forms. Indeed, the promising results in preclinical models set up the basis for ongoing clinical trials. Another two clinical trials to treat myotubular myopathy by MTM1 gene therapy or tamoxifen repurposing are also ongoing. Here, we review the contribution of the different CNM models to understanding physiopathology and therapy development with a focus on the commonly dysregulated pathways and current therapeutic targets.

Hepatobiliary disease in XLMTM: a common comorbidity with potential impact on treatment strategies

BACKGROUND

X-linked myotubular myopathy (XLMTM) is a rare congenital myopathy resulting from pathogenic variants in the MTM1 gene. Affected male subjects typically present with severe hypotonia and respiratory distress at birth and they often require intensive supportive care. Long-term survivors are often non-ambulant, ventilator and feeding tube-dependent and they generally show additional organ manifestations, indicating that myotubularin does play a vital role in tissues other than muscle. For XLMTM several therapeutic strategies are under investigation. For XLMTM several therapeutic strategies are under investigation including a study of intravenous MTM1 gene transfer using a recombinant AAV8 vector of which has some concerns arises due to hepatotoxicity.

RESULTS

We report prospective and retrospective clinical data of 12 XLMTM patients collected over a period of up to 10 years. In particular, we carried out a thorough review of the data about incidence and the course of hepatobiliary disease in our case series.

CONCLUSIONS

We demonstrate that hepatobiliary disease represents a common comorbidity of XLMTM that seems irrespective to age and diseases severity. We recommend to carefully explore and monitor the hepatobiliary function in XLMTM patients. We believe that a better understanding of the pathogenic mechanisms that induce hepatobiliary damage is essential to understand the fatal events that may occur in the gene therapy program.

Genetic therapy for congenital myopathies

PURPOSE OF REVIEW

There has been an explosion of advancement in the field of genetic therapies. The first gene-based treatments are now in clinical practice, with several additional therapeutic programs in various stages of development. Novel technologies are being developed that will further advance the breadth and success of genetic medicine.Congenital myopathies are an important group of neuromuscular disorders defined by structural changes in the muscle and characterized by severe clinical symptoms caused by muscle weakness. At present, there are no approved drug therapies for any subtype of congenital myopathy.In this review, we present an overview of genetic therapies and discuss their application to congenital myopathies.

RECENT FINDINGS

Several candidate therapeutics for congenital myopathies are in the development pipeline, including ones in clinical trial. These include genetic medicines such as gene replacement therapy and antisense oligonucleotide-based gene knockdown. We highlight the programs related to genetic medicine, and also discuss congenital myopathy subtypes where genetic therapy could be applied.

SUMMARY

Genetic therapies are ushering in an era of precision medicine for neurological diseases. Congenital myopathies are conditions ideally suited for genetic medicine approaches, and the first such therapies will hopefully soon be reaching congenital myopathy patients.

Respiratory care in myotubular myopathy

X-linked myotubular myopathy is a neuromuscular condition caused by pathogenic variants in the MTM1 gene, which encodes for myotubularin, a phosphatidylinositol 3-phosphate phosphatase. Affected individuals typically require intensive medical intervention to survive, though there are some milder phenotypes. To date, respiratory management has been primarily supportive, optimising clearance of airway secretions, providing ventilatory support and prevention/early intervention of respiratory infections. Encouragingly, there has been significant progress in the development of novel therapeutic strategies such as gene therapy, enzyme replacement therapy and drugs that modulate downstream pathways. In this review, we discuss the common respiratory issues using four illustrative real-life cases, and summarise recent translational research, which offers hope to many patients and their families.

Class II phosphatidylinositol 3-kinase isoforms in vesicular trafficking

Phosphatidylinositol 3-kinases (PI3Ks) are critical regulators of many cellular processes including cell survival, proliferation, migration, cytoskeletal reorganization, and intracellular vesicular trafficking. They are a family of lipid kinases that phosphorylate membrane phosphoinositide lipids at the 3′ position of their inositol rings, and in mammals they are divided into three classes. The role of the class III PI3K Vps34 is well-established, but recent evidence suggests the physiological significance of class II PI3K isoforms in vesicular trafficking. This review focuses on the recently discovered functions of the distinct PI3K-C2α and PI3K-C2β class II PI3K isoforms in clathrin-mediated endocytosis and consequent endosomal signaling, and discusses recently reported data on class II PI3K isoforms in different physiological contexts in comparison with class I and III isoforms.

The Phenotype and Genotype of Congenital Myopathies Based on a Large Pediatric Cohort

BACKGROUND

Congenital myopathies (CMs) are a clinically and genetically heterogeneous group of hereditary muscular disorders. The distribution of genetic and histologic subtypes has been addressed in only a few cohorts, and the relationship between phenotypes and genotypes is only partially understood.

METHODS

This is a retrospective cross-sectional data collection study conducted at a single center. The clinical, histopathological, and molecular characterization of 104 patients with CM is reported.

RESULTS

The most common histopathological subtype was core myopathy (42%). Patients with severe endomysial fibrosis were more commonly unable to walk than patients with only a mild-grade endomysial fibrosis (56% vs 16%). Inability to walk was also more prevalent in patients with severe fatty replacement (44% vs 19%). The genetic etiology was more frequently identified among those patients with "specific" histologic findings (74% vs 62%). A definite molecular diagnosis was reached in 65 of 104 patients (62%), with RYR1 (24/104) and TTN (8/104) being the most frequent causative genes. Neonatal onset occurred in 56%. Independent ambulation was achieved by 74%. Patients who walked late were more likely to become wheelchair-dependent. Respiratory support was needed in one of three patients. Gastrostomy placement was required in 15%. Cardiac involvement was observed in 3%, scoliosis in 43%, and intellectual disability in 6%.

CONCLUSIONS

This study provides an updated picture of the clinical, histopathological, and molecular landscape of CMs. Independently of the causative gene, fibrosis and fatty replacement in muscle biopsy specimens are associated with clinical severity. Mutations in TTN are responsible for a higher proportion of cases than previously thought.

The molecular mechanisms mediating class II PI 3-kinase function in cell physiology

The phosphoinositide 3-kinase (PI3K) family of lipid-modifying enzymes plays vital roles in cell signaling and membrane trafficking through the production of 3-phosphorylated phosphoinositides. Numerous studies have analyzed the structure and function of class I and class III PI3Ks. In contrast, we know comparably little about the structure and physiological functions of the class II enzymes. Only recent studies have begun to unravel their roles in development, endocytic and endolysosomal membrane dynamics, signal transduction, and cell migration, while the mechanisms that control their localization and enzymatic activity remain largely unknown. Here, we summarize our current knowledge of the class II PI3Ks and outline open questions related to their structure, enzymatic activity, and their physiological and pathophysiological functions.

From Mice to Humans: An Overview of the Potentials and Limitations of Current Transgenic Mouse Models of Major Muscular Dystrophies and Congenital Myopathies

Muscular dystrophies are a group of more than 160 different human neuromuscular disorders characterized by a progressive deterioration of muscle mass and strength. The causes, symptoms, age of onset, severity, and progression vary depending on the exact time point of diagnosis and the entity. Congenital myopathies are rare muscle diseases mostly present at birth that result from genetic defects. There are no known cures for congenital myopathies; however, recent advances in gene therapy are promising tools in providing treatment. This review gives an overview of the mouse models used to investigate the most common muscular dystrophies and congenital myopathies with emphasis on their potentials and limitations in respect to human applications.

Whole-genome analysis of noncoding genetic variations identifies multiscale regulatory element perturbations associated with Hirschsprung disease

It is widely recognized that noncoding genetic variants play important roles in many human diseases, but there are multiple challenges that hinder the identification of functional disease-associated noncoding variants. The number of noncoding variants can be many times that of coding variants; many of them are not functional but in linkage disequilibrium with the functional ones; different variants can have epistatic effects; different variants can affect the same genes or pathways in different individuals; and some variants are related to each other not by affecting the same gene but by affecting the binding of the same upstream regulator. To overcome these difficulties, we propose a novel analysis framework that considers convergent impacts of different genetic variants on protein binding, which provides multiscale information about disease-associated perturbations of regulatory elements, genes, and pathways. Applying it to our whole-genome sequencing data of 918 short-segment Hirschsprung disease patients and matched controls, we identify various novel genes not detected by standard single-variant and region-based tests, functionally centering on neural crest migration and development. Our framework also identifies upstream regulators whose binding is influenced by the noncoding variants. Using human neural crest cells, we confirm cell stage-specific regulatory roles of three top novel regulatory elements on our list, respectively in the RET, RASGEF1A, and PIK3C2B loci. In the PIK3C2B regulatory element, we further show that a noncoding variant found only in the patients affects the binding of the gliogenesis regulator NFIA, with a corresponding up-regulation of multiple genes in the same topologically associating domain.

De novo phosphoinositide synthesis in zebrafish is required for triad formation but not essential for myogenesis

Phosphoinositides (PIPs) and their regulatory enzymes are key players in many cellular processes and are required for aspects of vertebrate development. Dysregulated PIP metabolism has been implicated in several human diseases, including a subset of skeletal myopathies that feature structural defects in the triad. The role of PIPs in skeletal muscle formation, and particularly triad biogenesis, has yet to be determined. CDP-diacylglycerol-inositol 3-phosphatidyltransferase (CDIPT) catalyzes the formation of phosphatidylinositol, which is the base of all PIP species. Loss of CDIPT should, in theory, result in the failure to produce PIPs, and thus provide a strategy for establishing the requirement for PIPs during embryogenesis. In this study, we generated cdipt mutant zebrafish and determined the impact on skeletal myogenesis. Analysis of cdipt mutant muscle revealed no apparent global effect on early muscle development. However, small but significant defects were observed in triad size, with T-tubule area, inter terminal cisternae distance and gap width being smaller in cdipt mutants. This was associated with a decrease in motor performance. Overall, these data suggest that myogenesis in zebrafish does not require de novo PIP synthesis but does implicate a role for CDIPT in triad formation.

Zebrafish Models of LAMA2-Related Congenital Muscular Dystrophy (MDC1A)

LAMA2-related congenital muscular dystrophy (CMD; LAMA2-MD), also referred to as merosin deficient CMD (MDC1A), is a severe neonatal onset muscle disease caused by recessive mutations in the LAMA2 gene. LAMA2 encodes laminin α2, a subunit of the extracellular matrix (ECM) oligomer laminin 211. There are currently no treatments for MDC1A, and there is an incomplete understanding of disease pathogenesis. Zebrafish, due to their high degree of genetic conservation with humans, large clutch sizes, rapid development, and optical clarity, have emerged as an excellent model system for studying rare Mendelian diseases. They are particularly suitable as a model for muscular dystrophy because they contain at least one orthologue to all major human MD genes, have muscle that is similar to human muscle in structure and function, and manifest obvious and easily measured MD related phenotypes. In this review article, we present the existing zebrafish models of MDC1A, and discuss their contribution to the understanding of MDC1A pathomechanisms and therapy development.

Insights into wild-type dynamin 2 and the consequences of DNM2 mutations from transgenic zebrafish

Dynamin 2 (DNM2) encodes a ubiquitously expressed large GTPase with membrane fission capabilities that participates in the endocytosis of clathrin-coated vesicles. Heterozygous mutations in DNM2 are associated with two distinct neuromuscular disorders, Charcot-Marie-Tooth disease (CMT) and autosomal dominant centronuclear myopathy (CNM). Despite extensive investigations in cell culture, the role of dynamin 2 in normal muscle development is poorly understood and the consequences of DNM2 mutations at the molecular level in vivo are not known. To address these gaps in knowledge, we developed transgenic zebrafish expressing either wild-type dynamin 2 or dynamin 2 with either a CNM or CMT mutation. Taking advantage of the live imaging capabilities of the zebrafish embryo, we establish the localization of wild-type and mutant dynamin 2 in vivo, showing for the first time distinctive dynamin 2 subcellular compartments. Additionally, we demonstrate that CNM-related DNM2 mutations are associated with protein mislocalization and aggregation. Lastly, we define core phenotypes associated with our transgenic mutant fish, including impaired motor function and altered muscle ultrastructure, making them the ideal platform for drug screening. Overall, using the power of the zebrafish, we establish novel insights into dynamin 2 localization and dynamics and provide the necessary groundwork for future studies examining dynamin 2 pathomechanisms and therapy development.

The intragenic microRNA miR199A1 in the dynamin 2 gene contributes to the pathology of X-linked centronuclear myopathy

Mutations in the myotubularin 1 (MTM1) gene can cause the fatal disease X-linked centronuclear myopathy (XLCNM), but the underlying mechanism is incompletely understood. In this report, using an Mtm1 ^-/y disease model, we found that expression of the intragenic microRNA miR-199a-1 is up-regulated along with that of its host gene, dynamin 2 (Dnm2), in XLCNM skeletal muscle. To assess the role of miR-199a-1 in XLCNM, we crossed miR-199a-1 ^-/- with Mtm1 ^-/y mice and found that the resultant miR-199a-1-Mtm1 double-knockout mice display markers of improved health, as evidenced by lifespans prolonged by 30% and improved muscle strength and histology. Mechanistic analyses showed that miR-199a-1 directly targets nonmuscle myosin IIA (NM IIA) expression and, hence, inhibits muscle postnatal development as well as muscle maturation. Further analysis revealed that increased expression and phosphorylation of signal transducer and activator of transcription 3 (STAT3) up-regulates Dnm2/miR-199a-1 expression in XLCNM muscle. Our results suggest that miR-199a-1 has a critical role in XLCNM pathology and imply that this microRNA could be targeted in therapies to manage XLCNM.

Mortality and respiratory support in X-linked myotubular myopathy: a RECENSUS retrospective analysis

PURPOSE

Individuals with X-linked myotubular myopathy (XLMTM) who survive infancy require extensive supportive care, including ventilator assistance, wheelchairs and feeding tubes. Half die before 18 months of age. We explored respiratory support and associated mortality risk in RECENSUS, particularly among patients ≤5 years old who received respiratory support at birth; this subgroup closely matches patients in the ASPIRO trial of gene therapy for XLMTM.

DESIGN

RECENSUS is an international, retrospective study of patients with XLMTM. Descriptive and time-to-event analyses examined survival on the basis of age, respiratory support, tracheostomy use, predicted mutational effects and life-sustaining care.

RESULTS

Outcomes for 145 patients were evaluated. Among 126 patients with respiratory support at birth, mortality was 47% overall and 59% among those ≤5 years old. Median survival time was shorter for patients ≤5 years old than for those >5 years old (2.2 years (IQR 0.7-5.6) vs 30.2 years (IQR 19.4-30.2)). The most common cause of death was respiratory failure (66.7%). Median survival time was longer for patients with a tracheostomy than for those without (22.8 years (IQR 8.7-30.2) vs 1.8 years (IQR 0.2-not estimable)). The proportion of patients living without a tracheostomy was 50% at age 6 months and 28% at age 2 years. Median survival time was longer with provision of life-sustaining care than without (19.4 years (IQR 3.1-not estimable) vs 0.2 years (IQR 0.1-2.1)).

CONCLUSIONS

High mortality, principally due to respiratory failure, among patients with XLMTM ≤5 years old despite respiratory support underscores the need for early diagnosis, informed decision-making and disease-modifying therapies.

TRIAL REGISTRATION NUMBER

NCT02231697.

Novel roles of phosphoinositides in signaling, lipid transport, and disease

Phosphoinositides (PPIns) are lipid signaling molecules that act as master regulators of cellular signaling. Recent studies have revealed novel roles of PPIns in myriad cellular processes and multiple human diseases mediated by misregulation of PPIn signaling. This review will present a timely summary of recent discoveries in PPIn biology, specifically their role in regulating unexpected signaling pathways, modification of signaling outcomes downstream of integral membrane proteins, and novel roles in lipid transport. This has revealed new roles of PPIns in regulating membrane trafficking, immunity, cell polarity, and response to extracellular signals. A specific focus will be on novel opportunities to target PPIn metabolism for treatment of human diseases, including cancer, pathogen infection, developmental disorders, and immune disorders.

Identification of drug modifiers for RYR1-related myopathy using a multi-species discovery pipeline

Ryanodine receptor type I-related myopathies (RYR1-RMs) are a common group of childhood muscle diseases associated with severe disabilities and early mortality for which there are no available treatments. The goal of this study is to identify new therapeutic targets for RYR1-RMs. To accomplish this, we developed a discovery pipeline using nematode, zebrafish, and mammalian cell models. We first performed large-scale drug screens in C. elegans which uncovered 74 hits. Targeted testing in zebrafish yielded positive results for two p38 inhibitors. Using mouse myotubes, we found that either pharmacological inhibition or siRNA silencing of p38 impaired caffeine-induced Ca²⁺ release from wild type cells while promoting intracellular Ca²⁺ release in Ryr1 knockout cells. Lastly, we demonstrated that p38 inhibition blunts the aberrant temperature-dependent increase in resting Ca²⁺ in myotubes from an RYR1-RM mouse model. This unique platform for RYR1-RM therapy development is potentially applicable to a broad range of neuromuscular disorders.

Muscle cells have storage compartments stuffed full of calcium, which they release to trigger a contraction. This process depends on a channel-shaped protein called the ryanodine receptor, or RYR1 for short. When RYR1 is activated, it releases calcium from storage, which floods the muscle cell. Mutations in the gene that codes for RYR1 in humans cause a group of rare diseases called RYR1-related myopathies. The mutations change calcium release in muscle cells, which can make movement difficult, and make it hard for people to breathe. At the moment, RYR1 myopathies have no treatment.

It is possible that repurposing existing drugs could benefit people with RYR1-related myopathies, but trialing treatments takes time. The fastest and cheapest way to test whether compounds might be effective is to try them on very simple animals, like nematode worms. But even though worms and humans share certain genes, treatments that work for worms do not always work for humans. Luckily, it is sometimes possible to test whether compounds might be effective by trying them out on complex mammals, like mice. Unfortunately, these experiments are slow and expensive. A compromise involves testing on animals such as zebrafish. So far, none of these methods has been successful in discovering treatments for RYR1-related myopathies.

To maximize the strengths of each animal model, Volpatti et al. combined them, developing a fast and powerful way to test new drugs. The first step is an automated screening process that trials thousands of chemicals on nematode worms. This takes just two weeks. The second step is to group the best treatments according to their chemical similarities and test them again in zebrafish. This takes a month. The third and final stage is to test promising chemicals from the zebrafish in mouse muscle cells. Of the thousands of compounds tested here, one group of chemicals stood out – treatments that block the activity of a protein called p38. Volpatti et al. found that blocking the p38 protein, either with drugs or by inactivating the gene that codes for it, changed muscle calcium release. This suggests p38 blockers may have potential as a treatment for RYR1-related myopathies in mammals.

Using three types of animal to test new drugs maximizes the benefits of each model. This type of pipeline could identify new treatments, not just for RYR1-related myopathies, but for other diseases that involve genes or proteins that are similar across species. For RYR1-related myopathies specifically, the next step is to test p38 blocking treatments in mice. This could reveal whether the treatments have the potential to improve symptoms.

Mouse model of severe recessive RYR1-related myopathy

Ryanodine receptor type I (RYR1)-related myopathies (RYR1 RM) are a clinically and histopathologically heterogeneous group of conditions that represent the most common subtype of childhood onset non-dystrophic muscle disorders. There are no treatments for this severe group of diseases. A major barrier to therapy development is the lack of an animal model that mirrors the clinical severity of pediatric cases of the disease. To address this, we used CRISPR/Cas9 gene editing to generate a novel recessive mouse model of RYR1 RM. This mouse (Ryr1TM/Indel) possesses a patient-relevant point mutation (T4706M) engineered into 1 allele and a 16 base pair frameshift deletion engineered into the second allele. Ryr1TM/Indel mice exhibit an overt phenotype beginning at 14 days of age that consists of reduced body/muscle mass and myofibre hypotrophy. Ryr1TM/Indel mice become progressively inactive from that point onward and die at a median age of 42 days. Histopathological assessment shows myofibre hypotrophy, increased central nuclei and decreased triad number but no clear evidence of metabolic cores. Biochemical analysis reveals a marked decrease in RYR1 protein levels (20% of normal) as compared to only a 50% decrease in transcript. Functional studies at end stage show significantly reduced electrically evoked Ca2+ release and force production. In summary, Ryr1TM/Indel mice exhibit a post-natal lethal recessive form of RYR1 RM that pheno-copies the severe congenital clinical presentation seen in a subgroup of RYR1 RM children. Thus, Ryr1TM/Indel mice represent a powerful model for both establishing the pathomechanisms of recessive RYR1 RM and pre-clinical testing of therapies for efficacy.

Failure to identify modifiers of NEBULIN-related nemaline myopathy in two pre-clinical models of the disease

Nemaline myopathy is a rare neuromuscular disorder that affects 1 in 50,000 live births, with prevalence as high as 1 in 20,000 in certain populations. 13 genes have been linked to nemaline myopathy (NM), all of which are associated with the thin filament of the muscle sarcomere. Of the 13 associated genes, mutations in NEBULIN (NEB) accounts for up to 50% of all cases. Currently, the disease is incompletely understood and there are no available therapeutics for patients. To address this urgent need for effective treatments for patients affected by NM, we conducted a large scale chemical screen in a zebrafish model of NEB-related NM and an N-ethyl-N-nitrosourea (ENU)-based genetic screen in a mouse model of NEB exon 55 deletion, the most common NEB mutation in NM patients. Neither screen was able to identify a candidate for therapy development, highlighting the need to transition from conventional chemical therapeutics to gene-based therapies for the treatment of NM.

Summary: NEBULIN-related nemaline myopathy currently has no treatment. We attempted to uncover new avenues for therapy by performing modifier screens, which unfortunately failed to identify modifiers that improved disease relevant phenotypes.

PI3K isoforms in cell signalling and vesicle trafficking

PI3Ks are a family of lipid kinases that phosphorylate intracellular inositol lipids to regulate signalling and intracellular vesicular traffic. Mammals have eight isoforms of PI3K, divided into three classes. The class I PI3Ks generate 3-phosphoinositide lipids, which directly activate signal transduction pathways. In addition to being frequently genetically activated in cancer, similar mutations in class I PI3Ks have now also been found in a human non-malignant overgrowth syndrome and a primary immune disorder that predisposes to lymphoma. The class II and class III PI3Ks are regulators of membrane traffic along the endocytic route, in endosomal recycling and autophagy, with an often indirect effect on cell signalling. Here, we summarize current knowledge of the different PI3K classes and isoforms, focusing on recently uncovered biological functions and the mechanisms by which these kinases are activated. Deeper insight into the PI3K isoforms will undoubtedly continue to contribute to a better understanding of fundamental cell biological processes and, ultimately, of human disease.

The expanding spectrum of neurological disorders of phosphoinositide metabolism

Phosphoinositides (PIPs) are a ubiquitous group of seven low-abundance phospholipids that play a crucial role in defining localized membrane properties and that regulate myriad cellular processes, including cytoskeletal remodeling, cell signaling cascades, ion channel activity and membrane traffic. PIP homeostasis is tightly regulated by numerous inositol kinases and phosphatases, which phosphorylate and dephosphorylate distinct PIP species. The importance of these phospholipids, and of the enzymes that regulate them, is increasingly being recognized, with the identification of human neurological disorders that are caused by mutations in PIP-modulating enzymes. Genetic disorders of PIP metabolism include forms of epilepsy, neurodegenerative disease, brain malformation syndromes, peripheral neuropathy and congenital myopathy. In this Review, we provide an overview of PIP function and regulation, delineate the disorders associated with mutations in genes that modulate or utilize PIPs, and discuss what is understood about gene function and disease pathogenesis as established through animal models of these diseases.

Summary: This Review highlights the intersection between phosphoinositides and the enzymes that regulate their metabolism, which together are crucial regulators of myriad cellular processes and neurological disorders.

An In Vitro Model of Charcot-Marie-Tooth Disease Type 4B2 Provides Insight Into the Roles of MTMR13 and MTMR2 in Schwann Cell Myelination

Charcot-Marie-Tooth Disorder Type 4B (CMT4B) is a demyelinating peripheral neuropathy caused by mutations in myotubularin-related (MTMR) proteins 2, 13, or 5 (CMT4B1/2/3), which regulate phosphoinositide turnover and endosomal trafficking. Although mouse models of CMT4B2 exist, an in vitro model would make possible pharmacological and reverse genetic experiments needed to clarify the role of MTMR13 in myelination. We have generated such a model using Schwann cell-dorsal root ganglion (SC-DRG) explants from Mtmr13^−/− mice. Myelin sheaths in mutant cultures contain outfoldings highly reminiscent of those observed in the nerves of Mtmr13^−/− mice and CMT4B2 patients. Mtmr13^−/− SC-DRG explants also contain reduced Mtmr2, further supporting a role of Mtmr13 in stabilizing Mtmr2. Elevated PI(3,5)P₂ has been implicated as a cause of myelin outfoldings in Mtmr2^−/− models. In contrast, the role of elevated PI3P or PI(3,5)P₂ in promoting outfoldings in Mtmr13^−/− models is unclear. We found that over-expression of MTMR2 in Mtmr13^−/− SC-DRGs moderately reduced the prevalence of myelin outfoldings. Thus, a manipulation predicted to lower PI3P and PI(3,5)P₂ partially suppressed the phenotype caused by Mtmr13 deficiency. We also explored the relationship between CMT4B2-like myelin outfoldings and kinases that produce PI3P and PI(3,5)P₂ by analyzing nerve pathology in mice lacking both Mtmr13 and one of two specific PI 3-kinases. Intriguingly, the loss of vacuolar protein sorting 34 or PI3K-C2β in Mtmr13^−/− mice had no impact on the prevalence of myelin outfoldings. In aggregate, our findings suggest that the MTMR13 scaffold protein likely has critical functions other than stabilizing MTMR2 to achieve an adequate level of PI 3-phosphatase activity.

Ca2+-induced sarcoplasmic reticulum Ca2+ release in myotubularin-deficient muscle fibers

Skeletal muscle deficiency in the 3-phosphoinositide (PtdInsP) phosphatase myotubularin (MTM1) causes myotubular myopathy which is associated with severe depression of voltage-activated sarcoplasmic reticulum Ca²⁺ release through ryanodine receptors. In the present study we aimed at further understanding how Ca²⁺ release is altered in MTM1-deficient muscle fibers, at rest and during activation. While in wild-type muscle fibers, SR Ca²⁺ release exhibits fast stereotyped kinetics of activation and decay throughout the voltage range of activation, Ca²⁺ release in MTM1-deficient muscle fibers exhibits slow and unconventional kinetics at intermediate voltages, suggestive of partial loss of the normal control of ryanodine receptor Ca²⁺ channel activity. In addition, the diseased muscle fibers at rest exhibit spontaneous elementary Ca²⁺ release events at a frequency 30 times greater than that of control fibers. Eighty percent of the events have spatiotemporal properties of archetypal Ca²⁺ sparks while the rest take either the form of lower amplitude, longer duration Ca²⁺ release events or of a combination thereof. The events occur at preferred locations in the fibers, indicating spatially uneven distribution of the parameters determining spontaneous ryanodine receptor 1 opening. Spatially large Ca²⁺ release sources were obviously involved in some of these events, suggesting that opening of ryanodine receptors in one cluster can activate opening of ryanodine receptors in a neighboring one. Overall results demonstrate that opening of Ca²⁺-activated ryanodine receptors is promoted both at rest and during excitation-contraction coupling in MTM1-deficient muscle fibers. Because access to this activation mode is denied to ryanodine receptors in healthy skeletal muscle, this may play an important role in the associated disease situation.

Therapeutic Aspects in Congenital Myopathies

The congenital myopathies are a genetically heterogeneous and diverse group of early-onset, nondystrophic neuromuscular disorders. While the originally reported "classical" entities within this group - Central Core Disease, Multiminicore Disease, Nemaline Myopathy, and Centronuclear Myopathy - were defined by the predominant finding on muscle biopsy, "novel" forms with multiple, subtle, and unusual histopathologic features have been described more recently, reflective of an expanding phenotypical spectrum. The main disease mechanisms concern excitation-contraction coupling, intracellular calcium homeostasis, and thin/thick filament interactions. Management to date has been mainly supportive. Therapeutic strategies currently at various stages of exploration include genetic interventions aimed at direct correction of the underlying genetic defect, enzyme replacement therapy, and pharmacologic approaches, either specifically targeting the principal effect of the underlying gene mutation, or addressing its downstream consequences more generally. Clinical trial development is accelerating but will require more robust natural history data and tailored outcome measures.

X-linked myotubular myopathy: A prospective international natural history study

OBJECTIVES

Because X-linked myotubular myopathy (XLMTM) is a rare neuromuscular disease caused by mutations in the MTM1 gene with a large phenotypic heterogeneity, to ensure clinical trial readiness, it was mandatory to better quantify disease burden and determine best outcome measures.

METHODS

We designed an international prospective and longitudinal natural history study in patients with XLMTM and assessed muscle strength and motor and respiratory functions over the first year of follow-up. The humoral immunity against adeno-associated virus serotype 8 was also monitored.

RESULTS

Forty-five male patients aged 3.5 months to 56.8 years were enrolled between May 2014 and May 2017. Thirteen patients had a mild phenotype (no ventilation support), 7 had an intermediate phenotype (ventilation support less than 12 hours a day), and 25 had a severe phenotype (ventilation support 12 or more hours a day). Most strength and motor function assessments could be performed even in very weak patients. Motor Function Measure 32 total score, grip and pinch strengths, and forced vital capacity, forced expiratory volume in the first second of exhalation, and peak cough flow measures discriminated the 3 groups of patients. Disease history revealed motor milestone loss in several patients. Longitudinal data on 37 patients showed that the Motor Function Measure 32 total score significantly decreased by 2%. Of the 38 patients evaluated, anti-adeno-associated virus type 8 neutralizing activity was detected in 26% with 2 patients having an inhibitory titer >1:10.

CONCLUSIONS

Our data confirm that XLMTM is slowly progressive for male survivors regardless of their phenotype and provide outcome validation and natural history data that can support clinical development in this population.

CLINICALTRIALSGOV IDENTIFIER

NCT02057705.

Centronuclear myopathies under attack: A plethora of therapeutic targets

Centronuclear myopathies are a group of congenital myopathies characterized by severe muscle weakness, genetic heterogeneity, and defects in the structural organization of muscle fibers. Their names are derived from the central position of nuclei on biopsies, while they are at the fiber periphery under normal conditions. No specific therapy exists yet for these debilitating diseases. Mutations in the myotubularin phosphoinositides phosphatase, the GTPase dynamin 2, or amphiphysin 2 have been identified to cause respectively X-linked centronuclear myopathies (also called myotubular myopathy) or autosomal dominant and recessive forms. Mutations in additional genes, as RYR1, TTN, SPEG or CACNA1S, were linked to phenotypes that can overlap with centronuclear myopathies. Numerous animal models of centronuclear myopathies have been studied over the last 15 years, ranging from invertebrate to large mammalian models. Their characterization led to a partial understanding of the pathomechanisms of these diseases and allowed the recent validation of therapeutic proof-of-concepts. Here, we review the different therapeutic strategies that have been tested so far for centronuclear myopathies, some of which may be translated to patients.